Wave division multiplexing (WDM) or dense wavelength division multiplexing (DWDM) puts data from different sources together on an optical fiber. Each signal is carried simultaneously on a separate light wavelength. Using WDM, separate wavelengths or channels of data are multiplexed into a stream that is transmitted on a single optical fiber. As many as 80 channels of data may be transmitted this way. Each channel carries a time division multiplexed (TDM) signal.
Each channel is demultiplexed back into the original signal at the end of the optical fiber. Different data formats can be transmitted together at different data rates using WDM. For example, Internet packet (IP) data, synchronous optical network (SONET) data, and asynchronous transfer mode (ATM) data can all travel at the same time within an optical fiber.
All-optical WDM networks offer a novel solution for transmission of high bandwidth over large distances in a highly cost effective way. Future networks are expected to use the all-optical advantage in which data does not have to be converted back into electrical formats for regeneration, reshaping and conversion. In order to fully exploit the benefits of an all-optical network, there is a need for a cost effective component that will support signal regeneration and wavelength conversion. Regeneration should be performed in a network in order to correct distortions and noise accumulated by the signal after traveling through long spans of fibers, optical amplifiers, erbium-doped fiber amplifiers (EDFA), optical repeaters, and add/drop or cross connect stations.
Wavelength conversion is needed to prevent wavelength blocking in the network. Wavelength blocking occurs when two different users attempt to use the same wavelength in one port. Current technology for signal regeneration is based on optics-electronics-optics (OEO) transponders. These transponders regenerate the signal by converting the lightwave to an electrical signal in a receiver, cleaning the noise from the electrical signal, and re-transmitting the clean signal by imposing the clean bits on an optical modulator. The transponder may use a tunable laser as the new optical source and may perform wavelength conversion of the input signal.
Problems with OEO solutions include a high cost for high bit rates, non-transparency for bit rate, protocol or bit shape, and latency problems in long networks.